The recent trend for portable electronic apparatus has increased the requirement for high energy density rechargeable batteries. High energy density is also important for batteries used for electric vehicles.
Nickel hydroxide has been used for years as an active material for the positive electrode of alkaline batteries. Examples of such nickel-based alkaline batteries include nickel cadmium (Ni--Cd) batteries and nickel-metal hydride (Ni--MH) batteries.
In a NiCd cell, cadmium metal is the active material in the negative electrode. NiCd cells use a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte. The charge/discharge reactions at the negative electrode are controlled by the following reaction: ##STR1##
During charge, electrons are supplied to the negative electrode, whereby Cd(OH).sub.2 is reduced to Cd. During discharge, Cd is oxidized to Cd(OH).sub.2 and electrons are released.
The reactions that take place at the positive electrode of a Ni--Cd cell are also reversible. For example, the reactions at a nickel hydroxide positive electrode in a nickel cadmium cell are: ##STR2## At the positive electrode, Ni(OH).sub.2 is oxidized to NiOOH during the charge operation. During discharge, the NiOOH is reduced to Ni(OH).sub.2.
In general, Ni--MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen, and a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical potential across a Ni--MH cell, the Ni--MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical generation of hydroxyl ions: ##STR3## The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron.
The reactions that take place at the nickel hydroxide positive electrode of a Ni--MH cell are the same as for a NiCd cell: ##STR4##
Hence, the charging process for a nickel hydroxide positive electrode in an alkaline storage battery is governed by the following equation: ##STR5##
The charging efficiency of the positive electrode and the utilization of the positive electrode material is effected by the oxygen evolution process which is controlled by the reaction: ##STR6## During the charging process, a portion of the current applied to the battery for the purpose of charging, is instead consumed by the oxygen evolution reaction (7). The oxygen evolution of equation (7) is not desirable and contributes to lower utilization rates of the positive active material upon charging. One reason both reactions occur is that their electrochemical potential values are very close. Anything that can be done to widen the gap between them, lowering the nickel electrochemical potential in reaction (6) or raising the electrochemical potential of the oxygen evolution reaction (7), will contribute to higher utilization rates. It is noted that the electrochemical potential of the oxygen evolution reaction (7) is also referred to as the oxygen evolution potential.
Furthermore, the electrochemical potential of reaction (7) is more temperature dependent than that of reaction (6). At lower temperatures, oxygen evolution is low and the charging efficiency is high. However, at higher temperatures, the electrochemical potential of reaction (7) decreases and the rate of the oxygen evolution reaction (7) increases so that the charging efficiency of the nickel hydroxide positive electrode drops. High temperatures at the positive electrodes may be due to the external environment at which the battery is operated. They may also be due to the heat generated within the battery by oxygen gas recombination at the negative electrodes.
One way to increase the electrochemical potential of equation (7) is by mixing certain additives with the nickel hydroxide active material when forming the positive electrode paste. U.S. Pat. No. 5,466,543 to Ikoma, and U.S. Pat. Nos. 5,571,636 and 5,451,475 to Ohta et al discloses certain additives which improve the rate of utilization of the nickel hydroxide in a wide temperature range. The present invention discloses new additives which improve the high temperature utilization of nickel-based positive electrodes.